1. Field of the Invention
This invention relates to a torque detecting system for detecting torque acting on a rotational member such as a power transmission shaft of a motor vehicle, and more particularly to such a torque detecting system which detects the torque on the basis of the angle of torsion of the rotational member.
2. Description of the Prior Art
Recently there is an increasing demand for torque detection. For example, in order to precisely control torque distribution to the front wheels and the rear wheels in a four-wheel drive vehicle, the torque acting on each rotational member must be detected with a high accuracy.
There has been wide known a torque detecting system which detects the angle of torsion of the rotational member due to torque acting thereon and determines the magnitude of the torque on the basis of the angle of torsion. For example, at least a pair of rotary encoders are provided on the rotational member and the angle of torsion is measured on the basis of the difference in phase .DELTA.t of the detecting signals of the encoders, and the angle of torsion is converted to torque.
As such a rotary encoder, there have been known those using optical means and magnetic means. For example, there is disclosed, in Japanese Unexamined Patent Publication No. 62(1987)-239031, a torque detecting system using magnetic rotary encoders. However, in order to detect the angle of torsion by use of such rotary encoders, the rotational member must be provided with a slit, a gear or the like, which adds to size and weight of the system. Further mounting error is inherently produced when mounting the rotary encoders. Further when residual torsional strain of the rotational member accumulates, the relative positions of the rotary encoders gradually change and the torque detecting accuracy deteriorates with time.
The torque acting on a rotational member can also be detected by use a strain gauge or a magnetostrictive material. However use of the strain gauge encounters difficulties in bonding the strain gauge to the rotational member and taking out a signal from the strain gauge, and use of the magnetostrictive material is disadvantageous in that the magnetostrictive material must be bonded to the rotational member or the rotational member must be machined to form a groove or the like.
Thus there has been proposed in Japanese Unexamined Patent Publication No. 3(1991)-115940 a torque detecting system which detects the torque on a rotational member in the following manner.
In the torque detecting system, a pair of magnetic recording media such as magnetic disks are attached to the rotational member spaced from each other by a predetermined distance L in the longitudinal direction of the rotational member, a pair of magnetic heads are provided respectively opposed to the magnetic recording media, periodic signals which are the same in phase are recorded on the media by the heads while no load is acting on the rotational member, the periodic signals are reproduced by the heads when load is acting on the rotational member, a phase difference .DELTA.t between the reproduced signals is detected and the torque T on the rotational member is calculated according to the following formula on the basis of the phase difference .DELTA.t. EQU T=.pi..sup.2 Gd.sup.4..DELTA.t.(f/fo) No/16L (1)
wherein G represents the transverse modulus of the rotational member, d represents the diameter of the rotational member, f represents the frequency of the reproduced signal, fo represents the frequency of the recorded signal and No represents the rotational speed of the rotational member upon recording of the signal.
The torque detecting system can accurately detect the torque without change with time since the periodic signals can be rewritten any time. However the system gives rise to another problem that when the rotational speed of the rotational member fluctuates, the periodic signals cannot be recorded with accuracy and an error is produced in the calculated value of the torque. Generally the speed of a rotational member in the driving system of a vehicle frequently fluctuates and it is very difficult to suppress the fluctuation.